As is well known, most of time an AC-DC converter is in light-load or no-load condition such as standby mode and no charged batteries in a charger, and Blue Angel and Energy Star have some regulations to limit the power consumption under such conditions. Therefore, AC-DC switching converters having green mode are widely used in AC-DC power supplies.
FIG. 1 shows a conventional flyback AC-DC converter 100, in which a line voltage VAC is filtered by an EMI filter 102 and rectified by a bridge rectifier 104 to produce an input voltage Vin, a controller 108 switches a power switch SW such that a transformer 106 transforms the input voltage Vin to an output voltage Vo for a load RL, an auxiliary coil Laux of the transformer 106 provides a current Iaux to charge a capacitor C2 to produce a supply voltage Vcc connected to an input VCC of the controller 108, a photocoupler 112 generates a feedback signal FB from the output voltage Vo, an error amplifier 110 determines an error signal VEA connected to a feedback input COMP of the controller 108 by comparing the feedback signal FB with a reference voltage Vref. In the controller 108, an input CS receives a current sensing signal Vcs that is produced by a resistor Rs having the switched current in the power switch SW to flow therethrough, a comparator 116 determines a comparison signal S1 by comparing the error signal VEA with the current sensing signal Vcs, and a control logic 118 generates a pulse width modulation signal PWM to switch the power switch SW according to the comparison signal S1 and a clock CLK provided by an oscillator 114.
When the load RL of the converter 100 becomes light, the output power reduces dramatically, but the power loss in the circuit will not be eliminated, and thus the efficiency is lowered. Under light-load condition, switching loss dominants the power loss, and thus the key to improve the efficiency is to reduce the switching loss. Conventionally, when the converter 100 enters into a green mode, the error signal VEA will vary between two threshold values burst_high and burst_low, and when the error signal VEA is lower than the low threshold value burst_low, the power switch SW is turned on to pull up the error signal VEA until the error signal VEA becomes higher than the high threshold value burst_high.
Although operating with green mode can reduce the switching loss during light-load condition, when the load RL further reduces or even to no load, the controller 108 no longer turns on the power switch SW and thus the auxiliary coil Laux no longer supplies the current Iaux to charge the capacitor C2. Even though there is no power consumption of the power switch SW, the controller 108 still consumes power since its internal circuit always drains current, which lowers the supply voltage Vcc. Because the time constant of the capacitor C2 is much smaller than that of a capacitor C5 on the output Vo, the supply voltage Vcc will falls down to an undervoltage lockout cut-off threshold value UVLO and the converter 100 will enters into a hiccup mode. However, the load RL might suddenly change under the hiccup mode, thus it is better not to operate with a hiccup mode.
FIG. 2 shows a conventional method to prevent the supply voltage Vcc from being lower than the undervoltage lockout cut-off threshold value UVLO by setting a threshold value VCC_L for the supply voltage Vcc. When the supply voltage Vcc falls down to the threshold value VCC_L, the controller 108 will force to turn on the power switch SW to increase the current sensing signal Vcs, as shown by waveforms 200, 202, 204 and 212 of FIG. 2. However, in this circumstance the error signal VEA is nearly zero, and thus a minimum VEA_CLAMP is provided to clamp the error signal VEA, as shown by waveform 210. To prevent error operation during a green mode, the minimum VEA_CLAMP for the error signal VEA must be lower than the threshold values burst_high and burst_low as shown by waveform 206 and 208. By switching the power switch SW in such way, the supply voltage Vcc will remains at some level finally, as shown by the waveform 200. However, the minimum VEA_CLAMP for the error signal VEA must be a constant and lower than the threshold value burst_low during the green mode, and thus there is no enough energy as that during the green mode, resulting in the increase of switching times of the power switch SW and thereby the degradation of efficiency.
Therefore, it is desired a light-load efficiency improving method and apparatus for a flyback converter.